My car seems to have picked up a very fine but very long scratch (it goes from the front of the car to the back). If I was to guess I'd say it is likely to have been caused by a thorn on the spiky bush at the entrance to my drive.

What is the best way to remove this?

The scratch is very thin - I cannot get my fingernail in it. I'll try washing and polishing it tomorrow, but if that is ineffective what next?

The car is Monaco Blue so scratches show up rather badly unfortunately.

Scratch as above or if it is very fine autoglym super resin polish although this contains fillers so will partially mask it.If not it will easily go with some menzerna machine polishing compound on a hand applicator work in for about 4-5 minutes.Just rewax it afterwards as it will look slightly different to the rest of the panel without protection on it.If you cannot feel it with your nail it will come out.

If inflicted by a thorn bush it's only going to be the clearcoat layer that's scratched rather than the paint. A quick machine polish will get this back to mint. If you haven't got a polisher it's really worth investing in one to spritz up the paint once in a while and also for remedial work like this. I'd recommend getting a UDM or meguiars G220, you can do it yourself and they are designed for novices so little/no danger of damaging the paint.

Depends how deep the scratch is. If your fingernail can get caught in it then it will be impossible to remove completely (would mean machining the clearcoat to a dangerously low level) If your fingernail doesnt get caught in it then it can easily be machine polished, ideally with a rotary as opposed to a Random Orbit, simply because the polish is broken down better. A rotary polisher isnt for a novice though. Per earlier posts, I wouldnt recommend purchasing any machine unless you are likely to use it more regularly. Take it to a recognised machine polishing expert. You'll find one local to you via http://www.detailingworld.com

Did a wash and polish today with bog standard Turtle Wax and elbow grease. Whilst the scratch has not totally gone it is a lot less noticeable - you have to get fairly close and be looking for it.

Unfortunately the act of polishing the car (which I don't do nearly enough due to total lack of time) has revealed all manner of other dinks, scratches and stone chips All normal wear and tear in these days of motorway commuting and ever-bigger cars in ever-smaller parking spaces, but annoying nonetheless.

Next stop Auto Glym I think, as covering them is fine given that I'm selling the car in October-ish, then maybe a professional valet/detail just before I sell it.

BTW, are the likes of ChipsAway any good for stone chips and miniscule dents?